1. Field of the Invention
The present invention deals with a lighting unit designed for functional similarity to an incandescent light source in which the principal source of light is an arc lamp supplemented by a standby filamentary light source, and which includes a compact "high frequency" power supply unit operating from a conventional 120 volt 60 hertz source.
More particularly, the present invention deals with the operating network in the power supply unit, and with the optimization of the switching means included in the operating network for both filament and arc lamp operation.
2. Description of the Prior Art
The present invention is a product of efforts to produce an energy efficient and comparatively low cost replacement unit for the electrically inefficient incandescent lamp. With the costs of energy rising, a need has arisen for a lighting unit which converts electrical energy into light with greater efficiency. Recently, as disclosed in U.S. Pat. No. 4,161,672, smaller, low wattage, metal halide lamps having high efficiencies and light outputs comparable to home incandescent lamps have been invented. Such lamps are potential energy efficient replacements for the home sized incandescent lamp provided that convenient low cost provisions can be made for standby illumination when such lamps are being started and for supplying the diverse electrical requirements for the standby and principal light sources.
The power supply of the present lighting unit represents an outgrowth of earlier high frequency power supplies in which a ferrite transformer, then controlled for non-saturated operation, and a transistor switch were significant elements. Such power supplies are disclosed in U.S. Pat. No. 4,350,930 and U.S. application Ser. No. 305,653.
In the application Ser. No. 305,653, the power supply therein disclosed produces an initial sustained (8 sec.) period of dc filament energization by means of a first, SCR switch conducting current from the dc supply, followed by a short duration period (8 msec) of high frequency operation of a second, transistor switch. High frequency operation of the second, transistor switch, which is sustained (2 sec.) after arc lamp current is sensed, ignites the arc, and provides the necessary power to transition the arc to the point where the dc supply will sustain it. Meanwhile, the high frequency switch operation also energizes the standby filament. When the arc has transitioned, and switching operation has discontinued, the filament continues to be energized by its series connection through the arc lamp to the dc supply. As the voltage of the arc increases as the arc lamp warms up, the filament dissipates less power, and in the final run condition, the filament is much less incandescent and draws relatively little power. In the foregoing arrangement, the switching means required to provide for the initial dc operation of the filament were separate from the high frequency switching means used for both filament and arc lamp energization. The timing of the switching operation and the requirement of separate semiconductor switches tended to increase the parts count and circuit costs, while the circuit did attain the desired performance objective of reduced electromagnetic interference during starting.